Episode 1: Marooned on Mars
Yorp: These extremely friendly one-eyed martians are completely harmless! They cannot kill Keen directly, however, they can get very annoying. They have a tendency to "push" Keen along. This can cause him to fall into pits of spikes, fire, etc. If you want to be nice, you can paralyse them for a few seconds, by jumping on them. However, if you want to resort to violence, they only take one shot to kill.
Garg: These vicious, aggressive martians are perhaps the second most dangerous creature in this episode. Despite their appearence, they are actually quite smart. If you are on the same platform as them, they will charge at you at great speeds, so you will have to be quick to kill them. One shot will do it.
Butler "Tin Can" Robot: This is one of two robots created for menial work and guard duties. This robot moves fast along platforms and pushes Keen, a bit like the Yorps do, only with greater speed. Again, no serious threat, although they can get annoying, and there is no way to kill them.
Tank Robot: The second robot is somewhat like a tank (look at how it moves along). The robot it self is harmless, but beware! He carries a gun, and will shoot with no hesitation. Be careful, these guys can't be killed either.
Vorticon: Vorticons are controlled by the Grand Intellect, some mysterious being on the planet Vorticon VI. They are the most dangerous enemy in this episode. They are the most dangerous because of two reasons: Firstly, they very intelligent - they follow you, and they can also jump very high; and secondly they take four shots to kill. They normally "guard" the lost parts to the ship.
Green Dangly Stuff
Ice Cannon: This doesn't really harm you, but freezes Keen in an Ice Cube for a short period of time.